In aircraft and other structures in which two or more metal, composite or other materials are joined by fasteners, stress may induce distortions in the materials. It may be necessary or desirable to detect, assess and repair the distortions prior to continued service of the aircraft or other structure. Eddy currents induced in a material may be perturbed by the presence of distortions in the material. Therefore, distortions in a material may be detected and assessed by analyzing changes in a magnetic field created by the eddy currents in the material. Conventional probes may detect the magnetic fields created by eddy currents using inductive coils. However, the minimum detectable distortion dimensions using these conventional inductive probes may be undesirably large for some applications.
Giant magnetoresistance (GMR) sensors are sensitive to magnetic fields over a broad range of frequencies all the way down to 0 Hz. The sensitivity of an inductive coil is reduced as the frequency of the magnetic field which is measured by the inductive coil decreases. Inspection of distortions within or through thick, electrically conductive parts may require low-frequency magnetic fields due to the skin depth phenomenon in conductors.
Therefore, a nondestructive inspection (NDI) probe and method which utilize giant magnetoresistance (GMR)-based sensors to detect changes in a magnetic field which is created by eddy currents in an electrically-conductive material, with the capability to detect and assess the magnitude of distortions or flaws having a relatively small size in the material are needed.